A hard-disk drive (HDD) is a non-volatile storage device, which is housed in a protective enclosure, that stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the space between a read/write head and the surface of a magnetic-recording disk must be tightly controlled. To provide a uniform distance between a read/write head and the surface of a magnetic-recording disk, an actuator relies on air pressure inside the hard drive enclosure to support the read/write heads at the proper distance away from the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A read/write head therefore is said to “fly” over the surface of the magnetic-recording disk. That is, the air pulled along by a spinning magnetic-recording disk forces the head away from the surface of the magnetic-recording disk. When the magnetic-recording disk stops spinning, a read/write head must either “land” or be pulled away.
A write-head of an HDD records data onto the surface of a magnetic-recording disk in a series of concentric tracks. References markers may be recorded in each track of a magnetic-recording disk. These reference markers are referred to as servo information. To help properly position the write-head when writing data, an HDD employs a servo mechanical control loop to maintain the write head in the correct position using the servo information stored on the magnetic-recording disk. When a read-head reads the servo information (servo information being read may be referred to as a position-error signal, or PES), a relative position of the head may be determined by a servo processor to enable the position of the head, relative to the desired track, to be continually adjusted as necessary.
There are some occasions, particularly during start-up or during certain failure modes, when the servo system is not “locked,” which means that the servo information is not correctly identified and distinguished from data or other information written on the disk. Under these circumstances, the servo information cannot be read and the radial position of the head on the disk cannot be determined. During this time the actuator is uncontrolled. Establishing or re-establishing “servo-lock” is one of the highest system priorities in a hard disk drive. Once the servo information is correctly recognized it can be demodulated and decoded to provide an accurate radial position down to a small fraction of a single track.
Generally, servo information is written to, and therefore read back from, the magnetic-recording disk at a constant known frequency. The use of a single fixed frequency greatly facilitates fast recognition of the servo information as distinct from data. However, because the velocity of the rotation of the magnetic-recording disk is greater at the outer edge of the disk (denoted “OD,” for outer diameter) compared to the inner edge of the disk (denoted “ID,” for inner diameter), the amount of physical space used to store servo information on the magnetic-record disk increases with proximity to the OD.